Membrane filter

ABSTRACT

The filter membrane comprises a porous structure made of a material selected from sintered ceramics, sintered metals, microporous carbon, and microporous glass, and is characterized by the fact that its entire external surface, including the inside surface of the pores in the structure, is covered with a thin and continuous film of carbon or of an oxide. In a sintered aluminum membrane, the surfaces of pores are coated with a dense and continuous thin layer of zirconia which prevents the surfaces from being degraded in service. In the figure, curve 3 shows how the throughput of a membrane without a zirconia layer falls off over time, while curve 4 shows little change in the same membrane having such a layer.

This is a continuation of application Ser. No. 179,364, filed Apr. 8,1988 which is a continuation-in-part of application Ser. No. 101,927filed Sept. 28, 1987, both now abandoned.

REFERENCE TO RELATED APPLICATIONS

This invention relates to application Ser. No. 179,365, filed Apr. 8,1988 to Florence Falletti and entitled "A METHOD OF TREATING POLLUTEDOIL-IN-WATER EMULSIONS OF MICROEMULSIONS" and assigned to the commoncorporate assignee, now U.S. Pat. No. 4,865,742.

Cheap and effective filter membranes are known comprising a porousstructure made of a material selected from: sintered ceramics, sinteredmetals, microporous carbon, and microporous glass. The term "microporousglass" designates either a body obtained by heating a stack ofcalibrated glass particles, or else a body obtained by melting a mixtureof oxides, then segregating said mixture into two phases, andpreferentially dissolving one of the two phases by chemical attack, as,for example, when manufacturing VYCOR glass as described in the bookCHEMISTRY OF GLASS published by The American Ceramic Society--1985,pages 108 to 114.

When using a sintered ceramic, filter membranes are frequentlyconstituted mainly or exclusively by grains of sintered alumina.

The term "filter membrane" designates a porous structure having asurface layer with pores of well-defined diameter determining theseparation power of the membrane. Such a membrane is frequently formedon a macroporous support using one or more superposed microporouslayers.

Such a membrane is frequently formed on a support layer having largerpores and which may be constituted, for example, by grains of sinteredalumina either with or without the aid of a vitreous phase, inparticular using silicates. It has been observed that the surfaceactivity of these membranes is modified during the course of filtercycles. In particular, reactions with the suspension to be filtered anda reduction in operating throughput have been observed, and this appliesparticularly to cases where the solution or the suspension to befiltered includes proteins or phosphate type groups, or othermacromolecules having numerous reactive groups. This appliesparticularly to suspensions containing casein, and to alcoholicfermentation liquors for wine, cider, beer, etc. . . . .

For a membrane comprising several superposed layers, it is generally thesurface layer which has the smallest diameter pores and which thusperforms the filter function.

It has been observed that the performance in operation of such membranesdepends not only on the pore diameter of the surface layer, but also onchemical and physico-chemical interactions between the surfaces of thepores and the fluids to be filtered. It is therefore essential to matchthe nature of the surface with the fluid under consideration.

In the past it has been the practice either to make an assemblyconstituted by a macroporous support and one or more microporous layersin which the entire assembly is made of a material which is well-matchedto the fluid, or else to make an assembly comprising a macroporoussupport made of any convenient material together with one or more layersmade of a material which is well-adapted to the fluid.

This solution suffers from the major drawback of requiring a method ofmanufacturing a microporous layer constituted by a material which iswell-adapted to each particular fluid. When ceramics, metal, and porousglasses obtained by particle agglomeration are used, this means that:powders need to be prepared having a grain size which is carefullycontrolled as a function of the desired pore diameter; a homogeneoussuspension, i.e. in general a suspension which is well deflocculated,and having rheological characteristics which are well-adapted todeposition needs to be developed, as does a method of deposition; and anappropriate sintering temperature must be sought, which temperature willdepend on the size of the particles to be bonded by sintering, i.e. onthe pore diameter.

When using microporous glasses obtained by segregating a liquid anddissolving one of its phases, this requires the development of acomposition which can be segregated to give a first phase whosecomposition matches the fluid and a second phase which is soluble,together with accurate control of the segregation process so as toobtain a porous structure having the desired pore diameter after thesecond phase has been dissolved.

One object of the present invention is to provide filter membranes whichare well matched to each specific utilization in a manner which issimpler and cheaper.

A further aim of the present invention is to provide a membrane of thistype cheaply, having good filter properties and retaining them inservice longer than do prior art membranes with surfaces constituted byalumina, even if the solution or the suspension to be filtered containsgroups of one of the types mentioned above.

More particularly, the invention seeks to provide a ceramic filtermembrane whose porous structure, whose pore diameter and whose porosityare practically the same as those of prior art membranes based onsintered alumina, and whose surface activity is little changed afternumerous filtering and washing cycles.

SUMMARY OF THE INVENTION

The present invention, in one aspect, provides a filter membranecomprising a porous structure made of a material selected from: sinteredceramics, sintered metals, microporous carbon, and microporous glass,the membrane being characterized by the fact that the entire externalsurface thereof, including the surface inside the pores in saidstructure, is covered with a thin and continuous film of carbon or of anoxide selected from:

MgO, Al₂ O₃, SiO₂, TiO₂, Cr₂ O₃, MnO, Fe₂ O₃, CoO, NiO, CuO, ZnO, Ga₂O₃, GeO₂, TlO₂, Nb₂ O₅, MoO₃, RuO₂, PdO, CdO, SnO₂, La₂ O₃, HfO₂, Ta₂O₅, WO₃, PbO₂, Ce₂ O₃, and Bi₂ O₃, alone or in combination, and B₂ O₃,BaO and CaO mixed together with at least one of the above-specifiedoxides.

When said structure is constituted in conventional manner by grainswhich are fixed to one another by "bonded" portions of their surfacesleaving pores therebetween delimited by the remaining "exposed" portionsof their surfaces, only said exposed portions of the grain surfaces arecovered with said thin film which runs continuously from one grain tothe next.

The mechanical strength of the porous structure is therefore not indanger of being degraded by the presence of the film.

Further, if the corrosion resistance of the film with respect to thefluids to be filtered or the fluids used for washing the membrane isbetter than the corrosion resistance of said structure, then the filmacts as a protective film therefor.

The thickness of said film preferably lies between 2 nanometers (nm) and1000 nm. This allows the protective film to be both thick enough toprovide its isolation function while simultaneously being thin enoughfor temperature variations to give rise to stress at relatively lowlevels which do not lead to cracking and deterioration.

The invention is advantageously applied when the average pore diameterof the layer of the structure having the smallest diameter pores, i.e.the pores in the surface layer if there are several layers, lies betweenabout 0.02 microns and 15 microns.

The thickness of said film preferably lies between 0.01% and 10% of theaverage pore diameter in the layer of the membrane having the smallestdiameter pores. The porosity of the membrane is then substantially thatof its initial porous structure.

The present invention also provides a method of manufacturing such afilter membrane, said method comprising a step of manufacturing a porousstructure as defined above, said method being characterized by the factthat it includes a step of forming on said structure a thin andcontinuous film of carbon or of an oxide selected from:

MgO, Al₂ O₃, SiO₂, TiO₂, Cr₂ O₃, MnO, Fe₂ O₃, CoO, NiO, CuO, ZnO, Ga₂O₃, GeO₂, TlO₂, Nb₂ O₅, MoO₃, RuO₂, PdO, CdO, SnO₂, La₂ O₃, HfO₂, Ta₂O₅, WO₃, PbO₂, Ce₂ O₃, and Bi₂ O₃, alone or in combination, and B₂ O₃,BaO and CaO mixed together with at least one of the above-specifiedoxides.

Naturally the film may be made by any method which is suitable fordepositing a thin layer in the mass of a porous body. Known methodsincludes coating, vapor phase deposition, soaking, etc. . . . .

However, it is preferable for said step of forming said film to comprisestages specified below.

When the film is a carbon film made by carbonizing an organic material,the stages are as follows: preparing a solution containing:

an organic material selected for leaving a carbon-containing residuewhen heated in a non-oxidizing atmosphere, specific mention may be made,for example, of coal pitch, phenolic polymers, and furfurylic polymers;and

a solvent for said organic material; impregnating said porous structurewith said solution in such a manner as to cause all of the pores of saidstructure to be filled with said solution; and

heating said impregnated structure progressively up to about 800° C. to1500° C. in a non-oxidizing atmosphere in order to evaporate the solventand then decompose said organic material into a fraction which isremoved in the form of gas and a carbon-containing residue which remainsin the form of a continuous film.

When the film is a carbon film made by vapor phase deposition, saidstructure is maintained in a non-oxidizing atmosphere including ahydrocarbon gas at low pressure (<10⁻¹ bar) and at high temperature(1000° C. to 1500° C.), so that said hydrocarbon decomposes on cominginto contact with the surface of said structure and deposits a thin filmof pyrolytic carbon thereon.

When the film material is an oxide or a mixture of oxides, the stagesare as follows:

preparing a solution containing:

one or more organic precursors of the alcoholate or the acetylacetonatetype corresponding to the selected oxide or oxides;

a plasticizer, cross-linking agent of the triethyleneglycol ortriethanolamine type; and

a solvent constituted by an alcohol;

impregnating said porous structure with said solution so that the poresin said structure are filled with said solution; and

progressively baking said impregnated structure so as to eliminate allof the components of said solution other than the oxide(s) formed fromsaid precursor(s).

Such solution preferably contains 1% to 10% by mass oxide equivalent, 5%to 20% by mass plasticizer, and the remainder being alcohol. The alcoholsolvent is preferably the alcohol of the alcoholate, or isopropanol ifacetylacetonate is used.

Also preferably, said progressive baking stage itself comprises thefollowing steps:

drying in ambient air at substantially ambient temperature;

slowly raising the temperature to about 350° C., with the rate oftemperature rise being less than 5° C. per minute, at least over thosetemperature ranges in which gas is given off because of evaporation ordecomposition of the organic materials in the solution;

rising to a baking temperature for at least about 10 minutes; and

cooling.

The major advantages of the method compared with making a microporouslayer which is entirely constituted by the material which is matched tothe fluid to be filtered, are as follows:

A wide variety of membrane surface types may be obtained using a singlecomposition for the porous structure with only the pore diameter of saidstructure being modified, and by changing the composition of the film.It is much easier to change the nature of the surface by changing thestarting substances used for forming the film than it is to modify saidnature by developing on each occasion a microporous layer with theappropriate pore diameter and constituted by the appropriate mixture ofoxides.

When using the above-mentioned preferred method, the composition of thefilm can be modified simply by changing the nature of the organicprecursor or by mixing organic precursors of different metals.

The method of the invention enables said film to be obtained while usingbaking temperatures which are generally considerably less than thetemperatures which would be required for sintering prior art microporouslayers.

This is particularly advantageous for obtaining membranes in which thepore diameter of the filter layer is relatively large, e.g. 2 microns to15 microns, for which sintering temperatures may be as high as 1800° C.

A second aspect of the present invention provides a filter membranewhich is mainly or exclusively constituted by grains of sinteredalumina, the membrane being characterized by the fact that the aluminais covered in a thin and continuous layer of zirconia. In some cases,this layer may advantageously be stabilized, in particular by addingyttrium oxide Y₂ O₃ at a molar concentration of not more than 11°relative to the zirconia.

Zirconia is chemically more inert than alumina relative to mostsubstances to be filtered at it constitutes a layer which isolates thesubstance to be filtered from the underlying alumina.

Preferably, the structure of the membrane is constituted in known mannerby grains of alumina which are fixed to one another by "bonded" portionsof the grain surfaces while leaving pores therebetween which aredelimited by the remaining "exposed" portions of the grain surfaces.

This membrane is then more particularly characterized by the fact thatonly said exposed portions of the grain surfaces are covered in a thinlayer of zirconia, said layer being continuous from one grain toanother.

There is then no danger of the mechanical strength of the structurebeing reduced by the presence of the zirconia.

Preferably, the thickness of said layer of zirconia lies between 10nanometers and 1000 nanometers. This ensures firstly that the protectivelayer is thick enough to provide its function of isolation, and secondlythat the layer is flexible enough to be able to follow smalldeformations of the grains of alumina without breaking, in particularduring variations in temperature.

The invention is advantageously applicable to cases where the averagepore diameter of the layer having the smallest pores, i.e., the pores ofthe membrane, lies between about 0.1 microns and 10 microns. Thesesmallest pores are the pores of the layer which provides filtering,which layer may be supported by another layer having larger pores andproviding the mechanical strength of the assembly.

Preferably, the thickness of said layer of zirconia lies between 0.1%and 10% of the average diameter of the membrane pores. The porosity ofthe membrane is then substantially the same as the porosity of itsporous alumina structure.

The present invention also provides a method of manufacturing such afilter membrane, said method including a step of manufacturing amembrane structure which is mainly or exclusively constituted by grainsof alumina which are fixed to one another and which leave porestherebetween, said method being characterized by the fact that it thenincludes a step of forming a thin and continuous layer of zirconia onthe surface of the alumina.

In order to make this layer of zirconia, any appropriate method ofdepositing a thin layer in the bulk of a porous body may naturally beused. These known methods include coating, vapor phase deposition,soaking, . . . .

Preferably, however, said step of forming a layer of zirconia itselfcomprises the following steps:

making a zirconiferous solution comprising:

an alcoholate of zirconium;

a plasticizer or cross-linking agent for the film of zirconia, such astriethylene-glycol, with the weight of the plasticizer lying between 5%and 100% of the weight of the zirconium alcoholate; and

a solvent constituted by an alcohol, preferably the source of alcohol ofthe zirconium alcoholate, with the weight of said solvent lying betweenthree times and twenty times the weight of the zirconium alcoholate;

impregnating said alumina membrane structure and its optionalmacroporous support with said zirconiferous solution in such a manner asto cause the pores of said structure to be filled with said solution;and

progressively baking said impregnated structure so as to remove all thecomponents of said zirconiferous solution except for the zirconia whichis formed from the zirconium alcoholate.

Preferably said alcohol of said alcoholate and said solvent isn-butanol, in which case said alcoholate is zirconium n-butylate.

Also preferably, said progressive baking step itself comprises thefollowing steps:

drying at substantially ambient temperature in ambient air;

slowly raising the temperature up to about 350° C. with the rate of risebeing less than 5° Celsius per minute at least during those temperatureranges where gases are evolved due to evaporation or decomposition ofthe organic materials in the zirconiferous solution;

raising the temperature to a baking temperature lying between 350° C.and 1200° C.;

maintaining the baking temperature for at least about ten minutes; and

cooling.

BRIEF DESCRIPTION OF THE DRAWING

An implementation of the present invention lying within the scope givenabove is described in greater detail below by way of non-limitingexample and with reference to the accompanying diagrammatic figures.

FIG. 1 is a graph with two curves showing variation in throughput for aprior membrane and for a first filter membrane in accordance with theinvention, both being used in a conventional type of tangential flowfilter apparatus, with throughput being plotted up the y-axis in litersper hour and per square meter of membrane and with time being plotted inhours along the x-axis.

FIG. 2 shows similar curves for another prior art membrane and for asecond membrane in accordance with the invention.

Other characteristics and advantages of the present invention appearsfrom the following description of embodiments of membranes in accordancewith the invention together with the methods of manufacturing them, saidembodiments being given purely by way of non-limiting example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The starting material is a porous alumina structure constituted by amacroporous support whose pore diameter is about 15 microns having amicroporous layer of sintered alumina fixed thereon by sintering, withthe pore diameter in the microporous layer being about 0.2 microns andwith the thickness of the layer being about 40 microns.

A thin film of titanium oxide is to be made on said structure. To dothis, a coating solution is prepared comprising 36 grams (g) of titaniumtetraisopropoxide (Ti[OiPr]₄), 20 g of triethanolamine (N[CH₂ CH₂ OH]₃)and 70 g of isopropylic alcohol.

The tube of alumina is slowly immersed in said solution. After a fewseconds, the tube is removed and is subjected to drying for severalhours in ambient air. The tube is then baked using the following heatingcycle: slow temperature rise (0.5° C./min) up to 100° C.; a pause of 20min; followed by a rise to 700° C. at 1° C./min to 3° C./min. Thetemperature is then maintained for 40 min, and cooling is performed byswitching off the power supply to the oven.

For the other above-mentioned oxides, the procedure is the same as fortitanium oxide, and the various components of the coating solution areindicated together with the proportions thereof in Table I. Thefollowing abbreviations are used.

    ______________________________________                                        TEA          triethanolamine                                                  TEG          triethylene glycol                                               Acac         acetylacetonate or pentanedionate                                EtOH         ethanol                                                          iPrOH        isopropanol                                                      tPeOH        tertiopentanol                                                   OtPe         tertiopentanolate                                                EtO          ethanolate                                                       OEt          ethanolate                                                       OiPr         isopropoxide                                                     ______________________________________                                    

There follows an example when the film material is constituted by twooxides: CuO and TiO₂.

The solution used then contains:

    ______________________________________                                                8 g         Cu(OEt).sub.2                                                    11 g         Ti(OEt).sub.4                                                    78 g         EtOH                                                             14 g         TEG                                                       ______________________________________                                    

The method then continues using the steps described above.

In the following example, the film material is a glass comprising fiveoxides: SiO₂, B₂ O₃, Al₂ O₃, CaO, BaO; three of these oxides: B₂ O₃,CaO, and BaO, do not appear in Table I since they are incapable ofproducing a worthwhile film on their own.

The steps of the method are the same as before, and the initial solutioncomprises:

    ______________________________________                                        precursor:     19.3   g         Si(OEt).sub.4                                                1.1    g         B(OEt).sub.4                                                 7.3    g         AlAcac.sub.3                                                 5.5    g         CaAcac.sub.2                                                 0.7    g         BaAcac.sub.2                                  alcohol:       70     g         EtOH                                          plasticizer:   20     g         TEA                                           ______________________________________                                    

Two examples of making a carbon film are described below.

In the first example, an organic material is carbonized. To do this, a5% solution of coal pitch is prepared in toluene, the porous structureis soaked in said solution so as to be completely impregnated, it isdrained and heated slowly in a non-oxidizing atmosphere, initially up toabout 100° C. in order to evaporate the toluene, and then up to 1000° C.in order to carbonize the film of coal pitch which remains on thesurface of the porous structure. This provides a carbon film having athickness of about 1% of the pore diameter.

The second example uses vapor phase deposition. The porous structure isplaced in an enclosure which is evaporated and heated up to 1200° C. Amixture containing 10% methane and 90% argon is then inserted into theenclosure up to a total pressure of 100 millibars. The methane thendecomposes on coming into contact with the surface of the porousstructure and forms a thin and continuous film thereon of pyrolyticcarbon whose thickness increases progressively. The treatment is stoppedwhen said thickness reaches the desired value, for example 0.1 μm; theduration of the treatment is about one hour and depends on the shape andthe structure of the sample and on the type of the enclosure.

Naturally, the invention is not limited to the various methods ofpreparation mentioned above.

In a further embodiment, the starting filter tube is constituted by amacroporous alumina support (pore diameter=15 microns) having a40-micron thick macroporous (pore diameter about 0.2 microns) membrane(likewise of alumina) fixed thereto by sintering.

An n-butanol solution is made comprising 105 g/l (grams per liter) ofbutanol/zirconium n-butylate complex (Zr(OBu)₄, BuOH) and 70 g/l oftriethylene-glycol. The alumina tube is slowly immersed in saidsolution. After a few seconds, the tube is withdrawn and it is subjectedto drying in ambient air for several hours. The tube is then baked inaccordance with the following temperature cycle: slow temperature riseup to 350° C. (1.5° C./min), then rise to 750° C. at a rate of 2° C./minto 3° C./min. The temperature is then maintained for 30 min and coolingis then obtained by switching off the power supply to the oven.

The thickness of the resulting film of zirconia varies between 5% and10% of the average diameter of the pores in the layer having thesmallest pores, i.e. 10 nm to 20 nm (this applies to a soakingoperation).

In comparison with a prior art membrane constituted by the startingfilter tube, and under identical filter conditions, a very clearimprovement in filter performance is observed when filtering wine, ascan be seen in FIG. 1. Curve 1 represents the prior art membrane andcurve 2 represents the membrane in accordance with the invention.

In yet a further embodiment, the starting filter tube is made of aluminaand has pores which are 5 microns in diameter. An n-butanol solution ismade comprising 115 g/ of butanol/zirconium n-butylate complex(Zr(OBu)₄, BuOH), 60 g/l of triethylene-glycol, and 15 g/l of anhydrousyttrium nitrate. After immersing the tube as above, it is subjected toheat treatment: a slow rise up to 350° C. (at 1.5° C./min) followed by arise to 1000° C. (at a rate of 2° C./min to 3° C./min), pause at thistemperature for 30 min followed by natural cooling.

The thickness of the yttrified zirconia film is less than 1% of the porediameter, i.e. 50 nm (this applies to a soaking operation).

In comparison with a prior art membrane constituted by the startingtube, a very large improvement in performance is observed when filteringfermented molasses liquor (see FIG. 2 in which curve 3 represents theprior art membrane and curve 4 represents the membrane in accordancewith the invention).

The present invention with respect to the last two embodiments describedabove has the following advantages in particular:

all of the layers of the filter element are coated and not only thesurface layer which constitutes the membrane;

the geometrical structure of the porous medium is not altered;

it is applicable to different pore sizes and to filter elements ofdiverse shapes (tube, plane, multi-channel);

good performance in the face of temperature variation by virtue of thethinness of the film; and

improved filter performance.

It may also be observed that a membrane in accordance with the inventionprovides substantially the same performance as would be provided by amembrane made solely of zirconia (stabilized or not). However, azirconia membrane would be much more expensive to obtain because of thetemperatures and sintering time required and also because of the cost ofthe raw materials.

    ______________________________________                                                                               PLASTI-                                OXIDE   PRECURSOR                                                                            ALCOHOL      CIZER                                             ______________________________________                                        MgO     12     g Mg(OEt).sub.4                                                                            88 g EtOH   8 g TEA                               Al2O3   44.5   g AlAcac3    86 g iPrOH  7 g TEG                               SiO2    4      g Si(OEt)4   94 g EtOH   5 g TEA                               Cr2O3   27.6   g CrAcac3    86 g iPrOH  8 g TEG                               MnO     14.3   g MnAcac2    90 g iPrOH  6 g TEG                               Fe2O3   35.3   g FeAcac3    80 g iPrOH 12 g TEA                               CoO     32.8   g CoAcac3    83 g iPrOH 10 g TEA                               NiO     31     g NiAcac 2H2O                                                                              82 g iPrOH 10 g TEG                               CuO     19.8   g CuAcac2    85 g iPrOH  g TEA                                 ZnO     13     g ZnAcac2    84 g iPrOH 12 g TEA                               Ga2O3   19.6   g GaAcac3    80 g iPrOH 15 g TEG                               GeO2    10     g Ge(EtO)4   89 g EtOH   7 g TEA                               TlO2    7.4    g Tl(OEt)    75 g EtOH  18 g TEG                               Nb2O5   14     g Nb(OEt)5   79 g EtOH  15 g TEG                               MoO3    14.3   g MoO2Acac2  79 g iPrOH 13 g TEA                               RuO2    30     g RuAcac3    75 g iPrOH 15 g TEA                               PdO     12.5   g PdAcac2    87 g iPrOH  8 g TEA                               CdO     12     g CdAcac2    87 g iPrOH 10 g TEA                               SnO2    13.8   g SnAcac3    88 g iPrOH  7 g TEG                               La2O3   24     g LaAcac3    79 g iPrOH 12 g TEG                               HfO2    17.5   g Hf(OtPe)4  78 g tPeOH 15 g TEA                               Ta2O5   14.8   g Ta(OEt)5   79 g EtOH  13 g TEA                               WO3     10     g WAcac3     83 g iPrOH 12 g TEA                               PbO2    5      g PbAcac2    91 g iPrOH  6 g TEG                               Ce2O3   13     g CeAcac3    83 g iPrOH 12 g TEA                               Bi2O3   15     g BiAcac3    79 g iPrOH 14 g TEA                               ______________________________________                                    

What is claimed is:
 1. A filter membrane comprising a porous structuremade of a material selected from sintered ceramics and sintered metals,and formed by grains which are fixed one to another by bonded portionsof their surfaces leaving pores therebetween delimited by the remainingexposed portions of the surfaces of the said grains, wherein the entireexposed portions of the surfaces of the grains including the surfaceinside the pores of the porous structure are covered with a thin andcontinuous film of an oxide selected from the group consisting of ZrO₂,MgO, Al₂ O₃, SiO₂, TiO₂, Cr₂ O₃, MnO, Fe₂ O₃, CoO, NiO, CuO, ZnO, Ga₂O₃, GeO₂, TlO₂, Nb₂ O₅, MoO₃, RuO₂, PdO, CdO, SnO₂, La₂ O₃, HfO₂, Ta₂O₅, WO₃, PbO₂, Ce₂ O₃, and Bi₂ O₃, alone or in combination, or inmixture with B₂ O₃, BaO or CaO, wherein the thickness of said film liesbetween 2 nm and 1000 nm, wherein the average pore diameter of thesurface layer of said structure lies between 0.02 microns and 15microns, the porosity of the membrane being then substantially that ofthe initial porous structure.
 2. A membrane according to claim 1,wherein the thickness of said film lies between 0.1% of 10% of theaverage diameter of the pores in the layer of said structure having thesmallest diameter pores.